Avoid ref when using format! for perf

Clean up a few minor refs in `format!` macro, as it has a tiny perf
cost. A few more minor related cleanups.

Author: nyurik

library/core/src/fmt/builders.rs

@@ -1026,7 +1026,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
 /// assert_eq!(format!("{}", value), "a");
 /// assert_eq!(format!("{:?}", value), "'a'");
 ///
-/// let wrapped = fmt::FormatterFn(|f| write!(f, "{:?}", &value));
+/// let wrapped = fmt::FormatterFn(|f| write!(f, "{value:?}"));
 /// assert_eq!(format!("{}", wrapped), "'a'");
 /// assert_eq!(format!("{:?}", wrapped), "'a'");
 /// ```

library/core/tests/future.rs

@@ -56,7 +56,7 @@ fn test_join() {
 
         let y = String::new();
         let x = join!(async {
-            println!("{}", &y);
+            println!("{y}");
             1
         })
         .await;

library/proc_macro/src/lib.rs

@@ -1544,10 +1544,10 @@ impl fmt::Debug for Literal {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Literal")
             // format the kind on one line even in {:#?} mode
-            .field("kind", &format_args!("{:?}", &self.0.kind))
+            .field("kind", &format_args!("{:?}", self.0.kind))
             .field("symbol", &self.0.symbol)
             // format `Some("...")` on one line even in {:#?} mode
-            .field("suffix", &format_args!("{:?}", &self.0.suffix))
+            .field("suffix", &format_args!("{:?}", self.0.suffix))
             .field("span", &self.0.span)
             .finish()
     }

library/std/src/fs/tests.rs

@@ -683,7 +683,7 @@ fn recursive_rmdir_toctou() {
     let drop_canary_arc = Arc::new(());
     let drop_canary_weak = Arc::downgrade(&drop_canary_arc);
 
-    eprintln!("x: {:?}", &victim_del_path);
+    eprintln!("x: {victim_del_path:?}");
 
     // victim just continuously removes `victim_del`
     thread::spawn(move || {

library/std/src/io/error.rs

@@ -775,7 +775,7 @@ impl Error {
     ///
     /// impl Display for MyError {
     ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    ///         write!(f, "MyError: {}", &self.v)
+    ///         write!(f, "MyError: {}", self.v)
     ///     }
     /// }
     ///